Digital Satellite Downlink Reception - American Forces Radio and ...

Defense Media Center Satellite Handbook V.3.26
Reflective surfaces come in several different shapes and sizes but are most
common in the parabolic or offset shape. Offset shaped antennas are nothing
more than a small section of the original parabolic antenna see figure 4-2. The
larger the reflective service the better defined the focal point becomes and
therefore more gain can be expected. The reflector sometimes mistakenly called
the antenna is the first step in a well-engineered system that will continue to
provide service under harsh
environments. If the size of
your dish is too small for the
signal you intend to capture,
nothing is going to compensate
for that. Working with an
analog signal you could get by
with a smaller dish but suffer
with a noisy picture. A digital
signal on the other hand is
perfect or nothing situation and
with a marginal or less
reflective surface you can
expect nothing.
Many of the small aperture Kuband
dishes sold these days
use an offset antenna, see
figure 4-2, a feedhorn design
which places the focal point
below the front and center of
the dish. This type of antenna,
Figure 4-2 An offset satellite antenna
as defined earlier is actually a
small oval subsection from a
much larger parabolic antenna design, is oval in shape with a minor axis (left to
right) that is narrower than its major axis (top to bottom). Because of its unique
geometry, the offset fed antenna requires a specially designed feedhorn, which
matches the antenna geometry precisely. For this reason, the offset fed antenna
and feedhorn are usually sold together as a single unit. This type of feed is called
a Low Noise Feed or LNF.
Amplifier “LNA/B/C/F”
The concentrated signal from the reflective surface is channeled to a low noise
amplifier that has a very low noise floor. The job for this section is to amplify the
signal to a level that is above the receiver’s threshold. The Low Noise Amplifier
(LNA) amplifies the signal at the output of the earth station’s antenna. The most
commonly used LNAs use gallium arsenide field effect transistors (GaAsFETs).
Typical noise temperatures of amplifiers produced today range from 15° K to 60°
K (LNB\C\F).
4-4